How to - Gardening Resources

Hardiness zones and frost dates

What is a hardiness zone?Canada has nine zones, ranging from 0 (the harshest) to 8 (the mildest). Each zone is divided into two parts, with “a” being harsher than “b.” In creating the zone map, minimum and maximum temperatures, snow cover, rainfall, wind patterns and the average number of frost-free days were all taken into account.

But as gardeners know, our weather patterns vary from year to year, especially with the effects of global warming. In addition, the location (south side versus north side) of your plot can affect the growing conditions of your plants.

How do you use it?As a general rule, if you live in Zone 6, you should have good success with plants hardy in Zone 6 and up to three zones colder. Plants hardy to Zone 8 would never survive the winter in your yard. So, although the zone map is a useful starting point for all of us, consider your own yard’s conditions and your area’s weather fluctuations when choosing yearly plantings.

About the Canadian Plant Hardiness Zone MapThe Canadian Plant Hardiness Zone Map was revised in 2000 (using weather data from 1961 to 1990). As well as plant survival data for trees and shrubs (the northern and southern extremes where a given species can survive and reproduce successfully), the effect of elevation and climate-change models were added to the equation.

At the nurseryIf you live in Zone 5, look for plants that are hardy to Zone 5 or lower (plant labels don’t differentiate between parts “a” and “b”). Perennials from Zone 6 or higher will likely be sold as annuals in your area, since they aren’t guaranteed to survive the winter. Ask your garden centre professional for help with queries and suggestions.